This invention relates to a respirator for use in replacing or assisting the respiratory function in patients in whom spontaneous respiration is either absent or insufficient. Such respirators are used in surgery and anaesthesia, in critical care and respiratory therapy units for resuscitation, and in the general nursing of patients, including infants, whose natural respiratory activity is inadequate. Respirators of this kind are sometimes also termed ventilators.
One known form of respirator is designed to deliver a respiratory gas volume to the lungs intermittently through an inlet tube of large (i.e. 2.5 cm) internal bore via an inspiratory valve, the expired gas being allowed to be discharged from the lungs through a similar outlet tube via an expiratory valve. The inspiratory circuit may contain a humidification device. The inlet and outlet tubes of the respirator are connected via a Y-piece to the patient duct, i.e. any attachment to the patient which delivers gas to the patient, such as an endotracheal tube, a mask, or a tracheostomy tube. The inlet and outlet tubes have a large bore to minimise resistance in the circuit.
The operation of the known respirators of this first kind is controlled in accordance with predetermined input parameters. In a so-called pressure-controlled respirator, respiratory gas under pressure is supplied to the patient and the inlet valve interrupts the supply of respiratory gas and the outlet valve is opened when a specific predetermined pressure has been built up in the patient duct. In a so-called volumetrically controlled system, a measured quantity of the respiratory gas is supplied to the patient via a nonreturn inlet valve and the outlet valve is controlled to connect the patient duct to atmosphere once the supply of respiratory gas has taken place. Respirators of this kind commonly have a safety valve in the inhalation line to relieve any excess pressure.
Various sophisticated control systems have been developed for use with these known respirators to improve their adaptation to the instantaneous physiological condition of the patient.
Whilst these control systems have improved the adaptation and safety of the respirators, the adaptation to the patient is still far from ideal and the intervention of a skilled human operator is vital to ensure adequate adaptation of the respirator to significant changes in the patient's condition. Especially when a patient is in a recovery situation, a complicated sensing and triggering system is required to synchronise the operation of the respirator with the patient's own spontaneous breathing effort. Even this system fails if the patient's respiratory frequency is high. Further, reversal of a patient from the effects of paralysing drugs, a change to manual respirator operation or a changeover to continuous positive airway pressure (CPAP) requires disconnection of the respirator.
Because of the large internal volume of the tubing and humidifier, the known respirators of this kind for use with adults are not suitable for children or babies.
A further drawback of the known respirators under discussion is that their complexity and size preclude them being used in situations where portability is required. This complexity also means that significant effort may be required in maintaining the respirator at peak operating efficiency and in keeping the respirator in a clean condition. The complexity of these respirators both in manufacture and operation naturally mean that they are high cost items of equipment requiring expensive and skilled operators for safe use.
When the thoracic compliance of a patient is low, the known ventilators generate at normal respiratory rates high pressures in the lungs which may cause damage. To avoid such pressures, a higher frequency with smaller tidal volumes is desirable. The known ventilators of this first kind are unsuitable for this purpose because of their large internal compressible volumes.
In an attempt to overcome this problem, a second known form of respirator, the so-called jet respirator, has been developed. In a jet respirator, a proximal end of the patient duct is open to atmosphere and the respiratory gas is supplied to the patient duct as a high pressure jet through a nozzle inserted into the open proximal end of the patient duct. Jet respirators are however of limited application.
While this second form of respirator shares many of the disadvantages of the first kind of respirator discussed above, it has advantages in certain applications and has a particular advantage of employing no valves in the patient circuit, so that spontaneous respiration by the patient in a recovery situation is facilitated.
In known jet respirators, the high pressure respiratory gas is delivered in pulses which determine the inhalation phase of respiration, the exhalation phase occurring between pulses with the exhaled air passing to atmosphere through the open proximal end of the patient duct around the respiratory gas nozzle. The frequency of breathing and hence the minute volume can be varied by varying the frequency of the pulses of respiratory gas, although this adjustment is not quite as simple as appears at first and other settings may require compensating changes because of the high pressure condition of the respiratory gas in order to maintain a desired composition of the respiratory gas. Further, variation of the composition of the respiratory gas and the introduction of the anaesthetic gases, when required, is complicated by the fact that the respiratory gas is at high pressure.
As a result of the open proximal end of the patient duct, the high pressure respiratory gas tends either to leak out or to entrain atmospheric air and this is a further complication which has to be taken into account in adjusting the concentration of the constituent gases of the respiratory gas and the introduction of anaesthetic gases into the respiratory gas. The use of high pressure respiratory gas also presupposes the availability of all the required gases under pressure and, although this will commonly be the case in large medical establishments, it does place a considerable limitation on the environment in which the jet respirator can be used. Another limitation is imposed by the fact that it may not always be possible to supply the required minute volume of respiratory gas under all circumstances.
Because of its construction the jet respirator has the undesirable feature of being noisy in operation and may also cause trauma due to vibration of the patient duct in the patient's airway.